Abstrict A mass flow meter includes a hollow base pipe having an inlet adapted
to be attached to the discharge line of an air compressor or like
apparatus to be tested. The base pipe also has an outlet end which
is fitted with one of a plurality of convergent nozzles of differing,
known throat diameters. A pressure gauge is fitted to the base pipe,
upstream of the nozzle, to sense the pressure therein. The pressure
gauge has ring-shaped mass flow rate indicia thereon which are individually
calibrated to conform to each of the sized nozzles to permit direct
reading of mass flow rates as a function of pressure.
Claims What is claimed is:
1. A device for measuring the mass flow rate of a compressible
fluid delivered from a source comprising:
a hollow bored conduit means having an inlet end adapted to be
placed into communication with said source and a discharge end including
a converging nozzle means fitted therein, said nozzle having a throat
section defining a cross-sectional area of a known numeric value;
fluid pressure sensing means including a pressure gauge communicating
with the hollow bore of said conduit means in an upstream location
from said nozzle means;
locking collar means at the discharge end thereof adapted to detachably
receive the nozzle means, whereby nozzle means of differing cross-section
areas may be sequentially fitted to the device to obtain a mass
flow rate indication for each of said nozzle means; and
the gauge having pressure indicia thereon and a plurality of concentric
mass flow rate indicia rings each corresponding to a nozzle of different
cross-sectional area, whereby a single device enables direct reading
of both pressure and mass flow rate for each of the different nozzles.
2. The device of claim 1 wherein the compressible fluid is air
and the source is an air compressor.
Description BACKGROUND OF THE INVENTION
The invention relates generally to fluid pressure and flow rate
measuring devices and, more particularly, to apparatus for determining
mass flow rates of compressible fluids, such as air, discharged
from an air compressor, for example.
Heretofore, the experience of air compressor users, such as construction
contractors and the like, suggests that there is a need for a convenient
and relatively simple method for the determination of the pressure-mass
flow (in cubic feet per minute or "CFM") relationship
of compressors operating in the field. Oftentimes air compressors
become inefficient after long usage and/or poor maintenance which
can lead to unsafe and/or inefficient operation of the tools or
implements which are pneumatically operated thereby. The measurement
of the mass flow of compressible fluids, such as air, normally requires
that both the density of the fluid and the volumetric flow rate
be independently measured. However, by using the device of the present
invention, the flow rate may be quickly and easily measured in the
field by means of a pressure gauge or transducer, so calibrated
with various nozzle means that it provides both the pressure and
the mass flow rate.
The present invention provides a relatively fast and simple method
and apparatus for measuring compressor performance in the field
to permit owner/operators to periodically monitor the working output
of the compressors, which heretofore has not been practical or easily
accomplished.
SUMMARY OF THE INVENTION
The mass flow meter device of the present invention comprises a
tube-shaped base pipe which has inlet and outlet ends with a hollow
bore extending therethrough. The inlet end of the base pipe is adapted
to be attached to the discharge pipe of the air compressor to be
tested. The outlet end of the base pipe includes a threaded locking
collar, which is adapted to securely hold in place one of a plurality
of converging nozzles thereon. The nozzles each have formed therein
a convergent throat portion at its outlet end of a given diameter,
which is sized to produce a range of pressure and calculated mass
flow rate or CFM readings on a calibrated pressure/CFM gauge which
communicates with the bore of the base pipe, upstream from the nozzle.
The pressure gauge has a face plate with graduated values imprinted
thereon for reading the air pressure in psi. The face plate also
has a plurality of concentrically arranged ring-shaped, dial indicia
in graduated form for reading mass flow rates in CFM. Each ring
of CFM indicia has calculated values corresponding to a unique range
of pressures for each of the plurality of nozzles. For example,
when four nozzles of varying throat diameters are employed, the
face plate of the pressure gauge carries four concentric rings of
dial indicia for reading mass flow rate, wherein each ring corresponds
to one of the four nozzle diameters.
By using the known concept of choked nozzle flow (sonic) conditions,
for which the ratio of the absolute pressures upstream and downstream
of a nozzle should be equal or greater than 1.89 the flow rate
is measured by means of the pressure gauge, so calibrated that it
gives both the pressure and the mass flow rate for a given nozzle
area.
A pressure and mass flow rate reading is taken for each of the
four nozzles sizes and a simple graph can then be plotted showing
the actual performance characteristic of an air compressor in terms
of its actual discharge pressure versus mass flow rate in CFM.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other features and advantages of the present invention
will become apparent when reference is made to the accompanying
drawings and the detailed description, wherein:
FIG. 1 is a side elevationsl view of a mass flow meter constructed
in accordance with the present invention;
FIG. 2 is a fragmentary, sectional side view of a nozzle attached
to the discharge end of the device depicted in FIG. 1; and
FIG. 3 is a graph of a typical performance curve of the type which
is generated from readings obtained by the device of FIG. 1.
DETAILED DESCRIPTION OF THE INVENTION
In the operation of certain pneumatic tools, such as the unique
supersonic air excavation tool disclosed in copending U.S. patent
application Ser. No. 877280 of Aubrey C. Briggs, it is imperative
that a sufficient pressure and mass flow rate of air be delivered
to the tool in order for it to function in the supersonic regime.
If the air pressure and mass flow rate should fall below the specified
minimum values, the subject tool no longer can achieve supersonic
velocity and its efficiency as an excavation device is greatly diminished.
Through usual wear, rough handling and general abusive field conditions,
portable air compressors begin to leak with the passage of time
and should be monitored and replaced if they are employed in critical
work environments such as the above-referred to supersonic air excavation
tool application. Of course, in other more conventional applications,
such as with a pneumatic jackhammer, a substandard operating air
compressor will tend to lower the efficiency of that tool as well,
but to a lesser extent. It is therefore desirable in field operations
to be able to determine in a simple manner if the air compressor
is, in fact, operating at its rated capacity.
Referring to the drawings, the mass flow meter 2 of the present
invention permits the field evaluation of air compressors in a simple
and efficient manner, as will be described in greater detail below.
The mass flow meter 2 comprises a tube-shaped base pipe 6 which
has a bore 8 passing therethrough. The pipe 8 has a threaded inlet
end 10 which is adapted to receive a threaded quick-disconnect type
fitting thereon which, in turn, is adapted to attach to a like quick-disconnect
fitting on the exhaust hose of the air compressor (not shown). The
base pipe 6 is preferably constructed of a high strength material
which is also preferably noncorrosive such as, for example, a cast
and machined bronze alloy material.
A ring-shaped locking collar 12 is positioned at the discharge
end of the base pipe 6 and includes an internally threaded bore
14 which engages a threaded portion 16 formed on the end of pipe
6 FIG. 2. The outer surface of the locking collar 12 is preferably
knurled as shown in FIG. 1 to facilitate manual rotation thereof.
The mass flow meter 2 further includes a pressure gauge 4 or similar
pressure responsive device such as a transducer. The gauge 4 has
a hollow stem portion 24 which is threadably attached to a bore
26 which is formed through the sidewall of the pipe 6 so as to permit
the sensing portion of the pressure gauge to be in communication
with the interior of the bore 8 and, thus, respond to the air pressure
therein. The gauge 4 also carries a face portion which has imprinted
thereon a pressure dial 32 graduated in pounds per square inch (psi)
with numeric pressure indicia, for example, ranging from 0 to 160
psi, associated therewith. An indicator needle 30 is attached at
its base to a rotatable shaft 28 at the center of the gauge face,
which moves in a conventional manner in response to the air pressure
sensed within the bore 8. The face of the gauge 4 also has four
concentric CFM rings, 34 36 38 and 40 respectively, thereon with
numeric mass flow rate indicia (in cubic feet per minute) associated
with each. As will be explained hereinafter, each of the mass flow
rate CFM indicia rings 34 36 38 and 40 is calibrated to be read
with a particular nozzle 20 of a specific diameter "D"
dimension.
The mass flow rate meter 2 may be supplied conveniently as a kit
which includes, for example, four converging nozzles 20 wherein
each nozzle has a throat 21 with a diameter "D" of a dimension
set forth below in Table I. Each nozzle is preferably numbered and
also color coded for ease of identification in the field.
TABLE I ______________________________________ Nozzle Color "D"
Pressure Mass Flow No. Code (Inches) (psig) (cfm) ______________________________________
#1 Green 0.188 + .001 1.5-150 15-80 #2 Blue 0.265 + .001 1.5-150
30-160 #3 Red 0.325 + .001 1.5-150 45-240 #4 Black 0.375 + .001
1.5-150 60-320 ______________________________________
As seen in FIG. 2 the nozzles 20 are fitted at the outlet end
of the base pipe 6 snugly received by a flat seat 18 and held in
place by the locking collar 12. The nozzles 20 convergingly taper
from an inlet portion 19 having a diameter which is equal to the
inside diameter of the base pipe bore 8 and smoothly converges to
the nozzle throat 21 at the outlet end thereof. An extended edge
23 is also preferably formed on the outlet end of the nozzles 20
to protect the throat 21 from being nicked or otherwise marred during
handling. This is important because the exact dimensions of the
throat 21 must be maintained so that accurate pressure and mass
flow rate reading are generated. The ends 23 of the nozzle 20 may
be color coded in accordance with Table I so that a given color
identifies a given nozzle throat diameter. This same color coding
system is preferably carried over to the face of the pressure gauge
4 wherein the mass flow rate indicia on ring 34 for example, which
is marked "CFM1", is color coded in green to match nozzle
" No. 1" in Table I. In a similar manner, indicia ring
36 is printed in the color code blue, ring 38 in red, and ring 40
in black.
The sizes of the "D" diameter dimensions chosen for the
converging nozzles 20 were so selected to cover a normal range of
pressure and flow conditions that are frequently found in field
compressors, for example, pressures and flows of up to 150 psig
and 300 cubic feet per minute (CFM), respectively. Each nozzle presents
a specific cross-sectional area at the throat 21 which provides
a specific resistance against which the compressor must operate
at a particular flow and permits the measurement of that flow by
measuring the pressure upstream from the nozzle. This is possible
due to the relationship between the mass flow rate (CFM) in standard
cubic feet of free air under choked (sonic) conditions, varies directly
with the absolute total pressure upstream of the nozzle 20 and
with the area of the nozzle throat 21 and inversely with the square
root of the absolute temperature of the air upstream.
The condition may be expressed as follows: ##EQU1## where: m=mass
flow rate (cfm)
P.sub.o =absolute pressure upstream of nozzle
A.sub.t =area of nozzle throat
T.sub.o =absolute air temp. Rankine (or .degree.F.+460.degree.)
upstream of nozzle.
It is also known that at choked (sonic) conditions, the ratio of
the absolute pressures upstream and downstream of a nozzle should
be equal to or greater than 1.89.
Since the exact area of each nozzle 20 is known, the mass flow
rate for a given pressure value along the pressure gauge indicia
ring 32 can then be calculated and the CFM indicia rings 34-40 on
the gauge 4 are then calibrated accordingly using the specific nozzle
diameters of Table I for each.
In order to examine the performance characteristics of a compressor
using the mass flow meter 2 the nozzles 20 should be connected
one at a time in sequence to the discharge pipe of the compressor.
Each of the nozzles Nos. 1-4 in Table I provides both the resistance
against which the compressor has to operate at a particular flow.
A pressure and CFM reading is first taken for nozzle No. 1 and then
that nozzle is removed from the base pipe 6 and replaced by nozzle
No. 2 and similar readings are made. This sequence continues until
readings have been obtained for all four nozzles. A typical example
of the preformance curve obtained by this method for a 175 cfm -
100 psi rated compressor, that had been in the field for some time,
is shown in FIG. 3. A data point was obtained for each of the nozzles
in the manner described and plotted on the graph depicted in FIG.
3.
It is readily observed in FIG. 3 that the compressor subject to
testing has deteriorated somewhat since its actual performance curve
has fallen away from its rated capacity point shown on the graph.
The graph of data also indicates to the operator that the particular
compressor tested is of sufficient capacity to perform in the region
beneath the curve plotted thereon. This is of practical value since
the data points of FIG. 3 indicate that the specific compressor
tested still possesses sufficient capacity to operate the air excavation
tool mentioned above within the supersonic regime as required for
proper efficiencies, which usually calls for an air supply within
the ranges of about 105-125 cfm and 90-100 psi. As noted in FIG.
3 these required mass flow rates and pressures fall to the left
and beneath the performance curve plotted thereon, indicating that
the particular compressor will deliver the specified amounts.
In addition to measuring mass flow rates and pressures for positive
displacement air compressors and the like, my invention can be modified
to measure the performance of vacuum pumps. By using the same principle
of multi-nozzles having known cross-sectional areas, the nozzle
20 would be reversed in the end of base tube 6 so that the throat
portion 21 is adjacent to the bore 8 (not shown) and the end 10
of the tube 6 would be attached to the suction end of the vacuum
pump. It is also possible to further modify the device 2 to permit
CFM readings for absolute pressure ratios less than 1.89 that is,
for flow rates less than sonic in the throat or, in other words,
for a non-chocked condition. The mathematical relationship for this
condition is no longer the same as in the choked state but it can
be calculated and an appropriate CFM indicia ring for low pressure
flows could be prepared on a removable transparent template, for
example, and snapped into place over the face plate of gauge 4 as
required. |